Abstract During infection, part of the cellular environment rewired by HIV-1 is composed by endogenous retroviruses. Studies that have revealed the HIV mediated increase of HERV-K HML-2 proteins, underscored HERV-K potential as an excellent source of antigenic targets for anti-HIV therapy. We have found that during HIV-1 infection of primary CD4+ T-cells, the high expressing HERV-K HML-2 elements are those that are unable to autonomously produce proteins. This creates a conundrum about which are the elements that are at the root of HIV-1 dependent HERV-K HML-2 protein production and by what mechanism they achieve it. We will investigate the mechanism by which HIV-1 mediates HERV-K HML-2 protein upregulation during infection. Furthermore, the repetitive nature of HERV-K HML-2 family has undermined their study in relation to the effect they have on HIV-1, as conventional analysis systems are often insufficient. By combining HERV-K-specific RNA and protein expression analyses we have devised an approach that not only can accurately establish the HERV-K expression profile of different cell types, but also determines which of those expressed HERV-Ks produces proteins and peptides. The identity of the specific elements responsible for HERV-K HML-2 protein expression is essential to investigate the consequences of their expression on HIV-HERV-K interaction, as small differences in amino acid sequence have been demonstrated to have important repercussions on their function. Taken together, this proposal will contribute to a better understanding of the relation between HIV-1 infection and HERV-K protein production, and provide the tools to assess specific HERV-K HML-2 as potential HIV-1-dependent antigenic targets for antiviral therapy.